The present invention relates to a servo system in a data storage device and, in particular, to the demodulation of position error signals (PES) within the servo system.
A data storage device, such as a magnetic disc drive, stores data on a recording medium. The recording medium is typically divided into a plurality of generally parallel data tracks. In a magnetic disc drive, the data tracks are arranged concentrically with one another, perpendicular to the disc radius. The data is stored and retrieved by a transducer or "head" that is positioned over a desired data track by an actuator arm.
The actuator arm moves the head in a radial direction across the data tracks under the control of a closed-loop servo system based on servo data stored on the disc surface within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback servo signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system rotates the actuator arm to adjust the head's position so that it moves toward a desired position.
There are several types of servo field patterns, such as a "null type" servo pattern, a "split-burst amplitude" servo pattern, and a "phase type" servo pattern. A null type servo pattern includes at least two fields which are written at a known phase relation to one another. The first field is a "phase" or "sync" field which is used to lock the phase and frequency of the read channel to the phase and frequency of the readback signal. The second field is a position error field which is used to identify the location of the head with respect to the track centerline.
As the head passes over the position error field, the amplitude and phase of the readback signal indicates the magnitude and direction of the head offset with respect to the track centerline. The position error field has a null-type magnetization pattern such that when the head is directly straddling the track centerline, the amplitude of the readback signal is ideally zero. As the head moves away from the desired track centerline, the amplitude of the readback signal increases. When the head is half-way between the desired track centerline and the centerline of the adjacent track, the readback signal has a maximum amplitude. The magnetization pattern on one side of the centerline is written 180.degree. out of phase with the magnetization pattern on the other side of the centerline. Thus, the phase of the readback signal indicates the direction of the head position error.
To control the servo system a single position error value must be determined for each pass over the position error field. Typically, the magnitude of the position error value indicates the distance of the head from the track centerline, and the sign of the position error value indicates the direction of the head's displacement. The position error values are typically created by demodulating the readback signal associated with the position error field. Demodulation of the readback signal from the null pattern has, in the past, always been a synchronous process. In a synchronous process, the exact phase of the readback signal from the position error field is known from the phase field's readback signal because the phase field is written on the storage medium at a known and fixed phase relation to the position error field. A phase-locked loop (PLL) is typically used to acquire the phase of the phase field, and this phase information is used for demodulating the position error field. The phase field must therefore be sufficiently long to enable the PLL to lock on to the phase and frequency of the readback signal. For example, the phase field may be 3 times longer than the position error field.
In a servo sector scheme, with servo fields interleaved with data fields, long phase fields consume valuable data sectors on the storage medium. These data sectors could otherwise be used for storing data. As disc storage capacity requirements continue to increase, there is a continuing need for reducing the area consumed by servo data.
The present invention addresses these and other problems, and offers other advantages over the prior art.